A novel coding scheme for acoustic transponders and acoustic switches

The Acoustic Transponder and Acoustic Switch are remotely actuated equipments used by swimmers for navigation and remote control. Both equipments are actuated by a Portable Acoustic Interrogator which uses an acoustic code of eight serial 50 millisecond code tone bursts with each followed by 50 millisecond blanking tone bursts. The code tone bursts are combinations of two code frequencies and are selected to make eight total codes. Six codes are used to actuate Acoustic Transponders, and two are used for setting and resetting Acoustic Switches. The codes are received by hard limiting constant false alarm type circuits and decoded with digital logic circuits using codes stored in a read-only memory. Tests completed in April 1973 showed the equipment to work nearly 100 percent of the time in enclosed or open waters when not limited by measured sound velocity gradients. Also, the equipment has had no false actuations even in areas of high background noise.